1. Field of the Invention
The present invention is in the field of corona treatment devices and is directed more particularly to an improved treatment roll to be used as the electrode, and more particularly the ground electrode, in apparatus for the treatment of polymeric films.
2. The Prior Art
It is known that polymeric materials such as polyethylene films which are initially non-receptive to inks may be rendered printable by subjecting the same to corona discharge. By way of example, procedures and apparatus for the treatment of films by corona discharge are described in U.S. Pat. Nos. 2,802,085--Rothacker, Aug. 6, 1957; 2,859,480--Berthold et al, Nov. 11, 1958; 2,881,470--Berthold et al, Apr. 14, 1959; 3,397,136 Balogh, Aug. 13, 1968, as well as in an article entitled Guide to Corona Film Treatment appearing in the May 1961 issue of Modern Plastics.
In general, corona treatment involves passing a film to be treated through a corona discharge in an air gap between an electrode and a grounded roller supporting the film. Typically the roller is comprised of a metal substrate covered by a dielectric coating and the electrode, which may be of any form, is spaced a predetermined distance from the surface of the dielectric, the electrode being of a width or transverse extent generally coextensive with the width of the film being processed.
Conventional treater rolls have employed as the dielectric materials elastomeric or polymeric coatings, such a silicone rubber, hypalon, epoxy, etc. A further type of treater roll has employed a glass layer as the dielectric coating for the grounded metal substrate.
Although polymer dielectric coated treater rollers are relatively inexpensive, they are subject to rapid wear and frequent breakdown. The breakdowns, which may take various forms, are occasioned by a plurality of factors including the reaction of the rubber material to the ozone generated in the course of the treatment; the tendency of the rubber dielectric to develop pinholes, with resultant spark-through or arcing, and the deterioration and degradation of the rubber as a result of the heat generated as an adjunct to the treatment process.
The utilization of glass covered treater rolls has provided improved wear characteristics and resistance to spark-through but such rolls have other inherent drawbacks which militate against their widespread use. More particularly, the rollers have an extremely high initial cost and are fragile. Additionally, since the coefficient of thermal expansion of glass differs substantially from that of the metallic substrate, a glass coated roll must be operated within a relatively limited temperature range since the thickness of the coating must remain relatively small. Such limitations mandate that the power dissipation of the treatment apparatus be limited and, thus, the through-put of a treatment device utilizing a glass coated treatment roll must be retained at a relatively low level.
Adding to the expense of the glass covered roller is the fact that the rollers surfaces must be extremely smooth such that there is no air gap intervening between the film to be treated and the surface of the dielectric since, if such gap existed, a further corona would be developed in the gap and the reverse surface of the film would also be treated in that area or those areas coincident with the gap. Such surfaces are difficult to form in a glass coated roll.
Adding to the deficiencies of known prior art rolls is the current trend toward restricting the use of organic solvent based inks to avoid pollution. Whereas solvent based inks can be printed on polymeric films which have been treated in low power corona fields, the water based inks now favored require a deep or high density corona discharge to render the film suitably receptive.